The invention relates to a method of coherent demodulation for digitally processing a digitally modulated signal having a continuous phase and a constant envelope, the modulated term of the above phase is equal to the convolution product of the phase impulse response extending over a plurality of bit periods by the binary information transmitted in packets. The received signal is transposed in the baseband over two quadrature channels, converted into a digital signal and transferred to a signal processor which carries out the demodulation method.
This method can be applied to whatever phase modulation of the aforesaid type (GMSK, MSK, 2SRC, TFM, GTFM, . . . ), whose phase evolution law following a progressive variation is advantageous in that it reduces the frequency spectrum. Another advantage is that the transmitted energy is constant.
Modulation of the GMSK type, whose phase variation extends over the largest time interval (5 bit periods), presents the best spectral efficiency. Unfortunately, this results in a considerable augmentation of intersymbol interference.
The use of this narrow band modulation may be considered for many domains such as protected VHF and UHF communication systems, satellite transmissions or mobile radio-networks. The above advantages have caused the "Groupe Special Mobile" (GSM) of the CEPT to retain this narrow-band modulation to be used in the future digital Pan-European digital mobile network beginning in 1992.
The various prior-art demodulation processes implement differential or coherent methods.
The differential demodulation method is advantageous in that it is relatively simple but the performance in terms of error rates is very poor.
The coherent demodulation method presents better performance but it requires an additional circuit for the carrier phase recovery.
One of the weaknesses found with this type of demodulation resides in the use of conventional synchronizing methods which utilize phase locked loops for recovering the carrier and clock signals.
In fact, when a system that operates in the Time-Division-Multiple-Access (TDMA) mode or in the Frequency-Hopping (FH) mode in which the signal is subjected to fading caused by the channel, the resynchronization times of the analog loops become too long and reduce the useful duration of the signal (see U.S. Pat. No. 4,570,125 by R. B. Gibson and B. Hill).
The implementation of a method of coherent demodulation by means of digital signal processing presents as the main advantage the possibility of storing and processing the signal in packets for each of which a sequential operation is performed ending in a decision as to the transmitted binary information.
The first operation of the sequence consists of finding the beginning of the packet; this is the frame-synchronization. Then the bit-synchronization determines the decision instants and ensures the proper temporal adjustment of the matched filter. The role of this filter is to reduce the noise without degrading the useful information.
The last operation is extremely important: it concerns the estimation of the initial phase and the residual frequency offset.
The initial phase is a parameter which is not controlled in a transmission system. A faulty estimation of this parameter is disastrous for the error rate.
The residual frequency offset is the resultant of the frequency offset between the transmitter and the receiver and the frequency offset due to the Doppler effect. A faulty evaluation of this frequency offset results in decision errors with respect to the last bits of the packet, when the phase has been sufficiently rotated for causing such errors.
After estimating the initial phase and the residual frequency offset a compensation is effected and finally a decision is made with respect to the transmitted bits.
A digital demodulation method utilized for the 2SRC modulation was proposed in the article by LOUBATON and VALLET entitled: "Demodulation pseudo-coherente de signaux de type MSK adaptee aux transmissions en EVF" and published in the Revue Technique Thomson-CSF, Vol. 17, September 1985, No. 3, pp. 521-554.
In this method the following processing sequence is found: frame-synchronization by partial correlation; bit-synchronization by detection of the zero crossing of the differential phase; matched filtering; estimation of the residual frequency offset by Fast-Fourier-Transform of the squared samples; estimation of the initial phase by averaging; and phase compensation.
After simulation, the proposed algorithms have been found to adapt poorly to the GMSK modulation.
Actually, as a result of the intersymbol interference which can no longer be neglected, the determination of the bit-synchronization is degraded considerably in the presence of a residual frequency offset exceeding 200 Hz.
In addition, for estimating this frequency offset, the modulation can no longer be eliminated by effecting a squaring operation.
A demodulation technique for packet transmission by radio is given in the article by C. HEEGARD, J. A. HELLER and A. J. VITERBI, entitled: "A microprocessor-based PSK Modem for Packet Transmission over Satellite Channels" and published in IEEE Trans., Vol. COM-26, No. 5, May 1978, pp. 552-564.
Inspired by this technique, which only applies to a PSK type of modulation without intersymbol interference, the object of the invention is to obtain a synchronization which permits the coherent demodulation of modulation that results in intersymbol interference, even with a high noise level and a large residual frequency offset.